The invention relates to a method for the back-etching of tungsten-coated substrate surfaces in the production of large-scale integrated (LSI) circuits, in which the substrate is pressed against a cooled specimen holder during the back-etching through the use of a retaining ring disposed on the periphery or edge of the substrate.
In microelectronic components, the various conductive layers, for instance a doped silicon substrate, polysilicon or metal tracks, are separated from one another by insulators (such as SiO2). In order to obtain functional circuits, the individual conductive layers must be contacted with one another at suitable points. To that end, as a rule, holes, which are known as contact holes or vias, are etched into the insulators and then conductive material is introduced into the holes. In submicrometer technology, a suitable method that has come to be established is to fill those holes with tungsten that is chemically deposited from the gas phase (TCVD process). A known procedure is to first deposit a TCVD layer of tungsten hexafluoride WF6 and H2 or SiH4. That layer is deposited over a TiN or TiW underlay (adhesive layer, barrier) and grows both on the entire substrate surface and bottom and side walls of the hole, until in the case of the side walls the layer grows together, closing over. In a subsequent step, which is back-etching, the conductive layer previously deposited on the horizontal substrate surface is removed entirely again, so that in the ideal case the contact holes or vias remain, completely filled with conductive material.
In tungsten back-etching, it is necessary for the substrate to be pressed against a cooled specimen holder (pedestal) through the use of a retaining device, known as a retaining or clamp ring. Due to machining tolerances and surface roughness, a microscopic void or gap in fact results between the pedestal and the substrate and is detrimental to good substrate cooling. In order to obtain better cooling of the substrate, the void between the substrate and the pedestal is therefore filled with helium, which is supplied through the pedestal. In order to prevent helium from reaching the front side of the substrate and thus to assure that the substrate will not be forced away from the pedestal by the helium being fed in, the substrate is pressed against the specimen holder by a retaining ring. Heretofore, the retaining ring has been constructed in such a way that it completely surrounds the substrate. The back-etching step is typically carried out in a plasma containing fluorine and/or chloride, and the most important influences of the process parameters on various target variables are already known per se.
Through the use of that known method, not only is the goal to fill the contact holes or vias with conductive material but also to achieve a substrate surface that is free of tungsten residues after the tungsten back-etching. That requirement of the back-etching process of removing all of the conductive material from the substrate surface is important if short circuits between tracks being produced later are to be avoided. In order to achieve that freedom from tungsten, the back-etching step is delayed timewise. In other words, over-etching is carried out until the surface of the TiN or TiW underlay (adhesive layer, barrier) is free of tungsten residues. As a result of that over-etching, the adhesive layer is thinned and sometimes even removed completely locally, especially in the peripheral region. The etching products which are thus liberated precipitate onto the periphery or edge of the substrate. That precipitation is concentrated in a region of the substrate retaining device that comes directly into contact with the substrate. If the substrate is then placed in the atmosphere after the end of the back etching process, the precipitation mixes with the oxygen contained in room air. The mixture is also no longer removed in subsequent work steps (such as rinsing with water) that are contained in the further course of the process.
If further conductive or nonconductive layers are next applied over the entire surface, then in the region of the xe2x80x9cprecipitationxe2x80x9d or in other words redeposition, layer separations occur as a consequence of the strains in the layers which are applied. The undesired, problematic layer separations represent a xe2x80x9cparticle burdenxe2x80x9d for the substrate surface. The particle burden results in yield losses and substrate rejection.
After the tungsten back-etching, the xe2x80x9cprecipitationxe2x80x9d (redeposition) was heretofore removed by time-consuming rinses with water. The success of such water rinses was not replicable, and at the same time water rinses on metal layers should largely be avoided and restricted in duration. Process conversions in the over-etching phase that are intended to reduce the precipitation, and in-situ plasma cleaning steps that followed the tungsten back-etching process were likewise unsuccessful.
It is accordingly an object of the invention to provide a method for preventing redeposition of etching products onto substrate surfaces during a tungsten re-etching process in the production of LSI circuits, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods of this general type and which helps to avoid problems in the undesirable redeposition described above.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for back-etching of tungsten-coated substrate surfaces in the production of large-scale integrated circuits, which comprises pressing a substrate against a cooled specimen holder during back-etching with a retaining ring being disposed on an edge of the substrate and only locally retaining the edge of the substrate with the retaining ring at retaining locations distributed over the circumference of the retaining ring, the retaining locations being backup-free relative to etching products liberated in the back-etching, causing the etching products to flow past the retaining locations and be purposefully deposited outside the substrate surface.
In accordance with another mode of the invention, there is provided a method which comprises distributing at least three retaining locations extending in prong-like fashion on the retaining ring as far as the edge of the substrate.
In accordance with a further mode of the invention, there is provided a method which comprises distributing the retaining locations as retaining locations being tapered to a point or rounded in a curve at ends facing toward the substrate.
In accordance with an added mode of the invention, there is provided a method which comprises selecting a width, a height and a length of at least a few millimeters for the prong-like retaining locations, and in particular selecting the width and the height of the prong-like retaining locations to be in a range of approximately 2 mm.
In accordance with an additional mode of the invention, there is provided a method which comprises distributing the retaining locations as prong-like retaining locations being flattened toward ends facing toward the substrate.
In accordance with a concomitant mode of the invention, there is provided a method which comprises covering the edge of the substrate with clearance with an additional covering ring resting on the retaining ring.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for preventing redeposition of etching products onto substrate surfaces during a tungsten re-etching process in the production of LSI circuits, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.